Rotary waveguide attenuator having energy absorbing slots

ABSTRACT

ROTARY-TYPE VARIABLE WAVEGUIDE ATTENUATOR USING CURRENT INTERRPUTING SLOTS IN A CIRCULAR WAVEGUIDE FIELD WHICH IS ROTABLE WITH RESPECT TO THE MICROWAVE FIELD ORIENTATION AND CURRENTS CARRIED BY THE SECTION. LOSSY PARALLEL PLATE TRANSMISSION LINES ARE COUPLED TO THE SLOTS.

ROTARY WAVEGUIDE ATTENUATOR HAVING ENERGY ABSORBING SLOTS Filed May 26, 1969 Feb; 9, 1971 5 CQALE ET AL 5 Sheets-Sheet 1 INVENTOR. bI-1'/r: .;|Ir 1kIin S. c 0 4e BY IIOmL. 016

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MyM Attorneys 3,562,679 ROTARY WAVEGUIDE ATTENUATOR HAVING ENERGY ABSORBING SLOTS Filed May 26, 1969 F. s. COALE ET AL Feb. 9, 1971 5 Sheets-Sheet 2 mk n www m Feb. 9, 1971 F. s. COALE ETAL ROTARY WAVEGUIDE ATTENUATOR HAVING ENERGY ABSORBING SLOTS Filed May 26, 1969 5 Sheets-Sheet 3 J S medw 9 Tm n od w E vfiW H 9 9 mm g I n.m I. i F F Fwfl F w 0 a, i

Feb. 1971 5 COALE ET AL ROTARY WAVEGUIDE ATTENUATOR HAViNG ENERGY ABSORBING SLOTS Filed May 26, 1969 .5 Sheets-Sheet 4 INVENTOR. F rank/in S. Coale William L 'WaIIick BY Attorneys Feb. 9, 1971 F, 5 CQALE E'TAL 3,562,679

ROTARY WAVEGUIDE ATTENUATOR HAVING ENERGY ABSORBING SLOTS 5 Sheets-Sheet 5 Filed May 26, 1969 Fig. ll

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E SLOT L .25 WID 125 WIDE SLOT ll H5 I2 I25 FREQUENCY IN GHZ I OWER TRANSMITTED W h B m w N l IS I m F m n m o m 5 mm m L E %5 MS B mm m||mm m m m v o lo 20 9%" William wal/ick 14A, 2%; rrorneys United States Patent 01 ice 4 3,562,679 Patented Feb. 9, 1971 3,562,679 ROTARY WAVEGUIDE A'ITENUATOR HAVING ENERGY ABSORBING SLOTS Franklin S. Coale, Pasadena, and William L. Wallick,

Van Nuys, Califi, assignors to Systron-Donner Corporation, Concord, Califi, a corporation of California Filed May 26, 1969, Ser. No. 827,730 Int. Cl. H01p 1/22 U.S. Cl. 333-81 7 Claims ABSTRACT OF THE DISCLOSURE Rotary-type variable waveguide attenuator using current interrupting slots in a circular waveguide section which is rotatable with respect to the microwave field orientation and currents carried by the section. Lossy parallel plate transmission lines are coupled to the slots.

BACKGROUND OF THE INVENTION This invention relates to microwave transmission devices and particularly to waveguide attenuators of the variable type.

Heretofore, variable waveguide attenuators have commonly used resistive vanes which are moved with respect to the electric field within the device. As the vane increases its area of effective tangency with the field, currents are generated and absorbed in its resistive material to thereby provide the desired attenuation. In another type of device, a tuned stub was matched to the guide and its position varied to increase or decrease coupling into a resistive load terminating the stub. These devices have been inherently limited to low power handling capacity and have been difficult to cool adequately. While the rotary vane (resistive) attenuator is precise, it has not been capable of high power dissipation.

In many applications, it may be desired to vary the power output of a microwave source by attenuation rather than by control of the input drive in order to maintain a favorable signal-to-noise ratio. Previous variable attenuators have had limited ability to perform this function. There is, therefore, a need for a new and improved variable waveguide attenuator.

SUMMARY OF THE INVENTION AND OBJECTS In general, it is an object of the present invention to provide a variable waveguide attenuator which will overcome the above limitations and disadvantages to simultaneously provide both precision and power handling ability in a single unit.

Another object of the invention is to provide a variable waveguide attenuator of the above character which can be made as electrically precise as the previously known rotary resistive vane-type attenuator.

In general, the above objects are achieved by providing a rotary slot attenuator which can be considered as a dual of the rotary vane attenuator. Instead of a resistive card of the vane attenuator being positioned in the waveguide to react with the electric field, there are now provided longitudinal slots running substantially the length of the cylindrical waveguide section. The slots couple into parallel plate lossy transmission lines positioned on each side of the circular guide section and extending the length of the slot. As the position of the slot is rotated, it presents a resistance to circumferential currents in the walls of the circular waveguide section so that, for appropriately selected modes of transmission, a propagation is severely attenuated and the microwave energy is dissipated into the parallel plate transmission lines adjacent the slots.

Cooling means is provided for carrying away the heat generated in the lossy transmission line.

These and other objects and features of the invention will become apparent from the following description when taken in conjuction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a variable waveguide attenuator constructed in accordance with the present invention.

FIG. 2 is a cross sectional view of the attenuator of FIG. 1.

FIG. 3 is a cross sectional view taken along the lines 3-3 of FIG. 2.

FIG. 4 is a cross sectional view taken along the lines 4 4 of FIG. 3.

FIG. 5 is a cross sectional view taken along the lines 55 of FIG. 3.

FIG. 6 is a cross sectional view taken along the lines 6-6 of FIG. 3.

FIG. 7 is a cross sectional view taken along the lines 7-7 of FIG. 3 and showing particularly the cooling arrangement.

FIG. 8 is a diagrammatical view of the central waveguide section of the device of FIG. 1 showing current lines in relation to the position of the slots.

FIG. 9 is a diagrammatical view similar to that of FIG. 8 and showing the effect of movement of the slot position.

FIG. 10 is a schematic drawing in perspective of the slot attenuator of the present invention and showing the electromagnetic field components in relation to the slots.

FIG. 11 is a graph depicting theoretical and actual attenuation values as a function of frequency for an attenuator of the present invention.

FIG. 12 is a graph of attenuation as a function of frequency in X-BAND and also shows the effect of change in slot Width.

FIG. 13 is a graph showing power distribution in various portions of the attenuator of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The attenuator of the present invention generally comprises three waveguide sections, a central section 11 of circular waveguide mounted between end sections 13, 15. The end sections are held in fixed positions relative to a microwave signal transmission system by flanges 16. Each of the end sections 13, 15 consist of a rectangularto-circular transition, the rectangular end 17 of which conforms to TB rectangular waveguide at the flange while the circular end 19 corresponds to 'DE mode circular Waveguide. Each circular end is fit with a portion 21 of a choke joint which mates with the other portion 22 mounted to each end of the circular section 11. Those portions 21 of the choke joints mounted on the ends 19 of the end sections are rigidly interconnected together through a hollow housing 23 surrounding and spaced from the cylindrical section. The housing 23 serves to support and maintain the end sections in spaced relationship and in alignment on a common rectilinear axis, such that both the narrow walls and broad walls of the rectangular waveguide ends lie in common planes respectively.

Each end of the housing 23 includes a bearing support plate 25 and bearing 27 for mounting the inner portions 22 of the choke joints and thereby also carries the interposed waveguide section 11 to which they are attached. In this way, the circular waveguide section is held free for rotation in bearings 27 about an axis common with that of the end sections 13, 15. The use of the choke joints in this fashion to provide for rotational movement between adjacent waveguide sections is believed sufficiently well-known in the art that further explanation is unnecessary.

Means are provided for interrupting currents induced by electromagnetic energy traveling through the circular Waveguide and consists of a pair of diametrically opposed, longitudinally extending slots 31, 33 formed in the wall of the circular waveguide section and extending substantially its entire length. Each slot is longitudinally aligned parallel to the direction of waveguide section and terminates at each end in a series of reductions or graduated steps 35 which taper closed at each end to provide a better impedance match between the unloaded waveguide and loaded transmission lines to be described. Each slot opens into a parallel plate, high loss transmission line 37 which extends a substantial distance transversely away from the waveguide. The loss is established in the transmission line by forming the walls thereof with slabs 39 of micro- Wave high loss material and by terminating the end of the line away from the waveguide section with a cap 41 of similar high loss material. The slabs and exterior of the guide wall are provided with interlocking shoulders 43, 45 to maintain spacing between them and the top cap and an extension 47 having spaced parallel sides which fits between the side slabs so that taken together the slabs are supported in spaced, parallel position with their innerwalls aligned on and continuous with the edges of the respective slot. An example of a suitable high loss microwave load material is SiC in boron nitride binder manufactured by Norton Co. and sold as JUM-3 stone. The space between the innerwalls of the side slabs is air filled.

A suitable metal box or housing is mounted to the circular waveguide section and supports the slabs of the parallel plate transmission line and also serves as a heat conductor to cooling means, if such is provided.

Means are provided for shifting the angular position of the circular waveguide and slots relative to the induced currents in the circular waveguide wall and consists of suitable gearing, including a wheel gear 51 coaxially mounted about the waveguide section and an associated drive pinion or worm gear 53 which connects through the housing to a shaft and knob 55, rotation of which controls the relative angle of the circular section with respect to the end sections.

Referring to FIG. 8, the instantaneous current lines in the innersurface of the circular waveguide section operating in TE mode are shown. It will be noted that the currents induced in a wall are symmetrical about a plane bisecting the cylindrical section longitudinally through its axis so that the effect of each slot will be the same but will operate on opposite sides of the waveguide section. For slots in the position 0:0, it is seen that essentially none of the current lines are cut by these slots since the currents are axially directed in the region of interest. On the other hand, in FIG. 9, many of the current lines are circumferential (or azmuthal) and are cut when the slot is positioned at an angle 0 greater than zero. Obviously, the number of current lines cut is in direct proportion to the sine of the angle 0 of rotation of the circular waveguide section.

When these current lines are cut, they excite the respective slot and cause propagation of electromagnetic energy into the lossy transmission lines in proportion to the number being cut. Simultaneously, any microwave modes which excite circumferential currents in the walls which are cut by the slot are suppressed and the energy associated with that mode is dissipated in the associated lossy transmission line.

Since considerable energy is dissipated in each line 37, it is a particular advantage of the present invention that an especially effective means of heat removal can be provided. For high loss, high power operation, it is preferred that the housing walls surrounding each of the lossy transmission lines should be provided with coils 57 (FIG. 7) in thermal contact therewith and connected to a source of cooling fluid for circulating through the coils. Such may be provided by various means; however, brazing of thermally conductive metal tubing to the housing walls and interconnecting the same through flexible tubing 58 to an inlet 60 and outlet 62 mounted in the housing has been found satisfactory.

The end transition sections are identical except as to orientation so that a description of one will serve for both. Each includes a circular to rectangular tapered transition into the broad walls of which are incorporated elongate slots 64, 66 extending generally along the dimetion of the transition. The outer sides of these slots open into elongate slabs 68 of microwave lossy material enclosed in a thermally conductive cap 70 preferably provided with projecting fins 72 for air cooling of the slabs 68. By placing the slots in the broad walls, the normal TE mode of propagation is unaffected since it does not induce currents that intersect the slots. Other modes, however, will be suppressed, particularly the TE mode.

It has been found that the length of the slots 33 and 64 are not critical in the great majority of the design cases. For broad band operation, an approximate minimum length for slot 33 is about 7 wavelengths. The length of slot 33 could be shorter if restricted bandwidth of operation can be tolerated. By increasing the length of slot 33, the bandwidth of operation and the amount of calibrated attenuation will be increased. The maximum length required for slot 64 under any condition is about 3 wavelengths.

In further explanation of the operation of the rotary slot attenuator of the present invention, it will be helpful to use the many similarities between it and its dual of the rotary vane attenuator. In the latter, the vane interacts with the electric fields whereas, in the present invention, the slot interacts with the currents of magnetic fields. The analysis is greatly simplified if incident fields are broken up into two orthogonal TE modes or components-one of which induces currents perpendicular to the slots 31, 33 and another, orthogonal component, which induces currents parallel to the slots. One of these modes is greatly affected by the slot or vane and the other essentially passes through unaffected in a manner analogous to the operation of the rotary vane attenuator.

The input wave is designated by E, travels from rectangular to circular waveguide with an absorbing slot which has no effect on the incident wave. It then enters the rotary section and can be decomposed into two orthogonal components of magnitude E Sin 0 and E Cos 0, where 0 is the angle of the slot (see FIGS. 8 and 9) as suggested above. The E Sin 0 component is attenuated by the slot and the other component, E Cos 0, travels through the rotary slot section virtually unattenuated. This wave E Cos 0 can be further decomposed into two waves, E Cos 6 and E Cos 0 Sin 0, with respect to its orientation in passing through the circular to reotangular transition containing the fixed slot. The E Cos 0 Sin 0 wave is absorbed by the action of the fixed slot and associated lossy material leaving the component E Cos 6 at the output. The resulting attenuation produced A:2O log Cos 0 The attenuation formula A=-20 log Cos 0 is accurate only if the slot absorbs all of the power. For high values of attenuation or short length slots all of the power is not absorbed by the slot and some couples to the output. Due to the different propagation characteristics of the orthogonal modes, they may combine with variable phase at the output. This may cause some unwanted frequency response and marked departure from the simple attenuation formula.

Assume as to FIG. 10' an incident wave E which is decomposed into two orthogonal components E Cos 0 and E Sin 0. Choosing a suitable reference point at the end of the slot, whose length is L, we have E Cos and Er Sin 0 where e" '-':eand a, is the attenuation per unit length of the slot and A5 is the difiference of the phase factors between the two orthogonal waves. At the output only, the Cos 0 component of the wave transforms to the TE mode of the rectangular guide.

For maximum departure from Cos 0 attenuation dependence we have,

Cos Giza- Sin 0 the resulting attenuation is L= log (Cos Hie- Sin 9) For very accurate attenuation measurements it is necessary to have a very large slot attenuation. If, however, there is no attenuation in the slot, only the phase term is present and depending upon the propagation velocity a quarter or half wave plate phenomenon exists.

For high power application it is of interest to calculate the power dissipated in the various elements. The input transition from rectangular to circular waveguide with a slot on the top and bottom ideally dissipates no powerit is only there to stop some reflections from the rotary slot discontinuity and to prevent resonance buildup.

- The slot dissipates under ideal conditions of infinite slot loss E Sin 0. The output power is E Cos 0 so that the end slot in the circular to rectangular transition dissipates a power equal to E (lCos 0--Sin 0). FIG. 13 shows the fractional amount of power lost in each sec tion as a function of angle. The end slot dissipates a maximum of A of the incident power when 0:1r/4.

Since circular geometries with rectangular slots are inconvenient to handle mathematically, the circular guide operating in the TE mode is transformed to an equivalent rectangular guide operating in the TE mode. (The magnetic field in the region of the slot is the same if the slot is narrow.) The dimensions of the rectangular guide are chosen so that the two guides have the same cutoff frequency and the same power transmitted down the guide. The condition of equal cutoff wavelength, )t requires 2a 3.41 a, where a is the width of the rectangular guide and a is the radius of the circular guide.

The condition for equivalent power flow is satisfied by setting the power flow equal for the two modes and calculating the appropriate b, in terms of a.

After the relationship between the two guides is established, we can then concern ourselves with calculating the power lost into the slot and therefore the attenuation per unit length of the slot section. By decomposing the TE mode into two waves, one traveling down the guide and the other transverse to the guide, one can compute the energy lost in the slotted section. The results of this analysis show that the attenuation per unit length,

at, is:

where A is the guide wavelength in the TE mode of circular waveguide of radius a with a slot width of I2 and a load of b The thickness of the slot is L, B: 1r/)\ FIG. 12 shows the total attenuation of an 8" section of slot in the X-Band region as a function of frequency. A water cooled unit which was constructed for use in the 2.6-3.95 gHz. frequency band dissipated 1O kw. average power kw. peak) with an input VSWR less than 1.15 and an insertion loss of less than 0.1 db.

FIG. 11 shows the actual and theoretical limits of attenuation as a function of angle for a typical attenuator, when the maximum slot attenuation is only 30 db. Higher values of attenuation can be designed so that the theoretical curve can be approached arbitrarily close.

We claim:

1. A rotary slot attenuator comprising first and second waveguide transition sections, each of which has a rectangular waveguide end corresponding to TE mode and a circular end corresponding to TE mode, a circular waveguide section corresponding to TE mode, means joining said transition sections and said circular section together in series on a common axis such that the circular ends of said transition sections are coupled to the ends of said circular section and such that said circular section is rotatable about said axis with respect to said transition sections, means forming elongate slots along the broad walls of said transition sections, an elongate slab of microwave lossy material positioned outwardly adjacent each of said slots in said transition sections, means forming diametrically opposed elongate slots along the walls of said circular waveguide section, means forming a lossy parallel plate transmission line mounted outwardly of and in association with each of said last named slots such that the planes of said transmission lines are continuous with the edges of the respective slot and directed away from said circular section, the plates of said transmission lines being formed of microwave lossy material so that electromagnetic energy propagating through said slots and into said transmission lines is absorbed by said plates.

2. An attenuator as in claim 1 further including means for precisely shifting the angular position of the slot relative to the induced currents in the circular waveguide section.

3. An attenuator as in claim 1 further including caps of lossy microwave material terminating said transmission lines.

4. An attenuator as in claim 1 further including a thermally conductive housing for supporting each of said transmission lines associated with said circular waveguide section, and cooling means connected to said housing.

5. An attenuator as in claim 4 in which said cooling means comprises a coil means attached to said housing and adapted to transport a fluid coolant.

6. An attenuator as in claim 1 in which said elongate slots in said circular waveguide section are about 7 wavelengths long.

7. An attenuator as in claim 6 in which said slots in said transition sections are 3 wavelengths long.

References Cited UNITED STATES PATENTS 2,542,185 2/1951 Fox 333-8l(B) 2,877,434 3/1959 Farr et al. 333-l (B)X HERMAN KARL SAALBACH, Primary Examiner 'P. L. GENSLER, Assistant Examiner U.S. Cl. X.R. 333-98 

